1. Field of the Invention
This invention relates to a video signal processing apparatus, and more particularly, to an apparatus arranged to process a video signal including line sequential signals with a discontinuous point at every predetermined period.
2. Description of the Prior Art
Generally, in rearranging line sequential signals into simultaneous signals, the kinds of the line sequential signals must be discriminated one from another for every horizontal scanning period (hereinafter referred to as an H period). Further, in recording or transmitting these signals, they are arranged in such a signal form that permits them to be discriminated from each other. For example, in recording signals of two different kinds in a line sequential manner, these line sequential signals have been arranged to have a DC component offset or a frequency offset at intervals of a 2 H period; or to have a flag signal added at intervals of the 2 H period. However, in reproducing the line sequential signals which have been arranged in this manner, it sometimes becomes impossible for a reproducing system to accurately discriminate the kinds of signals due to the adverse effect of a drop-out or a transmission distortion. This problem will be understood from the following description of a reproducing system, by way of example, wherein a one-field portion of a video signal, including line sequential color difference signals which have a DC offset as a discrimination signal at intervals of a 2 H period is continuously reproduced in the form of a still picture.
FIG. 1 of the accompanying drawings shows, in a block diagram form the essential arrangement of the conventional reproducing apparatus of the above-stated kind. FIG. 2 is a waveform chart showing the waveforms of outputs obtained at the various points (a) to (g) indicated in FIG. 1. Referring to FIG. 1, a terminal t1 is arranged to receive line sequential color difference signals reproduced from a video signal. A terminal t2 is arranged to receive a horizontal synchronizing signal obtained from the reproduced video signal. The reproducing apparatus is provided with a sample-and-hold circuit 1; an amplifier 2 arranged to amplify the output of the sample-and-hold circuit 1; a comparator 3 arranged to compare the level of the output of the amplifier 2 with a predetermined level; a D type flip-flop (DFF) 4 arranged to receive the output of the comparator 3 as a data input and to be triggered by a fall in the horizontal synchronizing signal, which is indicated at (d) in FIG. 2; a delay line 6, the delay time of which is one horizontal scanning period (1 H=63.556 .mu.sec); a monostable multivibrator 7 arranged to be triggered by the horizontal synchronizing signal (d) and to form a signal of a as shown at (b) in FIG. 2; and change-over switches SW1 and SW2. The contact piece E of the switch SW1 is connected to a contact point A and the contact piece F of the switch SW2 to a contact point C when the output signal of the DFF 4 is at a high level. The contact piece E of the switch SW1 is connected to the other contact point B and the contact piece F of the switch SW2 to the other contact point D of the switch SW2 when the output level of the DFF 4 is low. By this switching operation, the line sequential color difference signals are rearranged into line simultaneous color difference signals and are respectively supplied to terminals t3 and t4.
Let us assume that the line sequential color difference signals include a color difference signal R (red)-Y (luminance) and another color difference signal B (blue)-Y (luminance) and that the center level of the color difference signal R-Y is higher than that of the other signal B-Y and has been recorded in a line offset manner. Assuming that the signal R-Y is being reproduced during a certain H period, the signal which is sampled and held by the fall of the output signal (b) of the monostable multivibrator 7 during this H period is at a high level. Therefore, the output signal (e) of the DFF 4, produced during a next H period, is at a high level. More specifically, when the output of the 1 H delay line 6 is the signal R-Y, the contact piece E of the switch SW1 is connected to the contact point A to allow the signal R-Y, which has been rearranged into a simultaneous signal, to be produced from the terminal t3. Meanwhile, the signal B-Y, which has been likewise rearranged into a simultaneous signal, is allowed to be produced from the other terminal t4.
With the line sequential color difference signals arranged to be rearranged into line simultaneous color difference signals in the manner as described above, if there is some flaw, such as a poor S/N ratio due to a drop-out or the like, in the line sequential color difference signal reproduced at the time of sampling, it is no longer possible to obtain a result of the sample-and hold process with a correct DC offset. As a result of that, an error arises in the switching operation of the switches SW1 and SW2. Then, the signals R-Y and B-Y would be conversely rearranged. For example, deterioration in the S/N ratio due to a drop-out, indicated by an arrow A, causes a converse rearrangement during a period B, as shown in FIG. 2. This is highly disagreeable on a reproduced picture as it appears, for example, in the form of a red line in the event of a blue monochromatic picture. In the case of a still picture reproducing apparatus, in particular, the S/N ratio deterioration due to a drop-out or the like is apt to arise during the same H period to result in a very conspicuous disagreeable line.